We have generated two transgenic mouse lines that allow us to evaluate the effects of elevated blood levels v/s high mammary levels of tissue inhibitor of metalloproteinases-1 (TIMP-1) on mammary carcinogenesis. Mammary carcinomas were induced either by oral administration of DMBA or crossing with Middle T transgenic mice which develop spontaneous metastatic mammary carcinomas. Tumor growth was reduced significantly by the elevated blood levels of TIMP-1 in both the DMBA and the Middle T carcinogenesis models. The incidence of lung metastases was also reduced. Analyses of the mammary carcinomas showed a decrease in the number of both proliferating and apoptotic tumor cells, but no change in vascular density. In contrast to the findings in the transgenic line with elevated blood levels of systemic TIMP-1, targeting of TIMP-1 to mammary epithelial cells under the control of the MMTV did not result in tumor growth suppression in either the DMBA or the Middle T mammary carcinogenesis models. Vascular density was not affected by the elevated TIMP-1 levels in the mammary carcinomas. In a group of carcinomas with very high TIMP-1 expression levels, there was a positive correlation between TIMP-1 levels and proliferative activity, shown both by PCNA staining and BrdU uptake vivo. We do not have an explanation why the stimulation of proliferative activity did not result in increased tumor growth. We conclude from these findings that continuous release of TIMP-1 into the systemic circulation for the duration of the carcinogenic period suppresses growth of mammary carcinomas. However, overexpression of TIMP-1 in mammary epithelial cells did not suppress growth and high TIMP-1 tumor levels were even associated with increased proliferative activity of tumor cells. These models emphasize the complexity of TIMP-1 functions in tumor development and progression. Data from phase III clinical trials with synthetic MMP inhibitors showed no benefit in patients with advanced breast cancer. Our findings suggest that elevated TIMP blood levels from the early stages of breast cancer development are effective in halting tumor growth. An in vitro study was carried out to characterize the molecular mechanisms involved in TIMP-1-mediated inhibition of endothelial migration, a critical step in the angiogenic process. TIMP-1 down-regulated phosphorylation of focal adhesion kinase (FAK) and paxillin, which resulted in disorganization of F-actin filaments. Although a general MMP inhibitors also inhibited endothelial cell migration, it did not affect phosphorylation of FAK or F-actin organization. Our data suggest that TIMP-1 can affect endothelial migration by both MMP-dependent and independent mechanisms, the latter involving suppression of FAK activity and cytoskeletal organization.